The subject of digital keyers and digital key signal generation has received considerable attention in the recent years. Several methods have been proposed and implemented by workers in this field, see for example: EP-A-0,360,518, EP-A-0,360,559, EP-A-0,360,557, EP-A-0,264,965, EP-A-0,360,560, EP-A-0,267,553, EP-A-0,236,943, EP-A-0,425,111 and U.S. Pat. No. 4,920,415.
`Keying` or overlaying of different video scenes is a common operation in video production. A key consists of three components:
(a) A background video signal, BGV; PA1 (b) A foreground video signal, FGV; and PA1 (c) A key signal, K.
As shown in FIG. 1 of the accompanying drawings these three signals are typically fed to a mixer 1. The key signal acts as the control signal for the mixer and can vary in the range 0.0 to 1.0 inclusive. When the key signal is equal to 1.0, the output of the mixer is foreground video. Similarly when the key signal is equal to 0.0, the mixer output is background video. When the key is between 0.0 and 1.0, the mixer output is a proportional mix of foreground and background videos.
A circuit known in the art for extraction of key signal from a foreground video signal is called a `Clip and Gain` circuit. A typical clip and gain circuit compares the level of the incoming video signal against a threshold level, called the Clip level. As shown in FIG. 2, if the video level is well below the clip level, the key signal value is zero. Similarly if the video level is well above the clip level, the key signal value is one. When the video level is equal to the clip level, the key signal value is 0.50. If the level of incoming video falls in a band centered by the clip level and with a width determined by the gain parameter (this is called the transition band), then the key signal value varies between 0.0 and 1.0. This is to ensure that there will be a soft transition from background video to foreground video in the constituted output video.
As the value of the gain parameter increases, the width of the transition band reduces and vice versa. High values of gain parameter cause a small transition band, meaning an abrupt switch at the mixer output between the foreground and background videos. Low gain values cause gradual transitions between the foreground and background videos. The clip and gain levels can be set for the desired effect.
A problem with keying techniques in digital video is the stair-stepping phenomenon. Each active line of the digital video signal is subdivided into pixels. A line of active video, in normal definition, is formed of 720 pixels (in High Definition the corresponding number is 1920). The visual result of imposing a discrete horizontal structure on a video signal is that a smoothly curved edge, will at best, be poorly represented. If a key signal is derived from this video signal, it will include a stair-stepping alias impairment of the original smoothly curved edge. This is due to the fact that the derived key signal will be forced to adopt transition points dictated by the nearest available pixel position to the desired transition position.
FIG. 3(a) shows video levels of a digital video signal. The dotted vertical lines indicate sampling time instances and crossings indicate the video level of each pixel. (Thus the sequence of video levels in this example is 0%, 0%, 0%, 0%, 10%, 90%, 100% and 100%. Note that the level of the video signal abruptly changes from one sample to the next (i.e. from sample 5 to 6).
Assuming that the clip level is set to 30%, a straight-forward technique for derivation of the digital key signal (i.e. implementation of the linear relationship between video signal level and key signal value shown in FIG. 2), would lead to the digital key signal shown in FIG. 3(b).
It is important to note that the desired 30% clip level has been totally lost. The key switching on a pixel-to-pixel basis has caused the stair-stepping phenomenon.
In contrast, in analogue video, this problem is less acute in that a corresponding analogue key signal,shown in FIG. 3C, could be derived such that the 50% crossing of the key signal happens at the time when the level of the original analogue video signal (shown by solid line in FIG. 3(a)), reaches 30%.
One way to improve the quality of the digital key signal is to increase the sampling frequency (i.e. use smaller pixels). A fast enough sampling frequency provides a more accurate representation of the transition waveform.
In High Definition video with the sampling frequency standing at a rate exceeding 70 MHz, a further increase of the sampling frequency is not currently technically possible.